X-ray fluorescence (XRF) measurement, and specifically X-ray microfluorescence (i.e., X-ray fluorescence using focused excitation beams of small diameter, such as less than 100 μm), is commonly used in testing semiconductor wafers. XRF itself is a well-known technique for determining the elemental composition and other properties, such as thickness, of a sample. XRF analyzers generally include an X-ray source, which irradiates the sample with sufficient energy to excite X-ray fluorescence from the elements of interest within the sample, and an X-ray detector, for detecting the X-ray fluorescence emitted by the sample in response to the irradiation. Each element in the sample emits X-ray fluorescence in energy bands that are characteristic of the element. The detected X-ray fluorescence is analyzed to find the energies or, equivalently, the wavelengths of the detected photons and the number of emitted photons (intensity) as a function of energy or wavelength, and the qualitative and/or quantitative composition of the sample is determined based on this analysis.
U.S. Pat. No. 6,108,398, for example, whose disclosure is incorporated herein by reference, describes an XRF analyzer and a method for analyzing a sample. The analyzer includes an X-ray beam generator, which generates an X-ray beam incident at a spot on the sample and creates a plurality of fluorescent X-ray photons. An array of semiconductor detectors is arranged around the spot so as to capture the fluorescent X-ray photons. The analyzer produces electrical pulses suitable for analysis of the sample.
The use of X-ray microfluorescence for testing semiconductor wafers is described in U.S. Pat. No. 6,351,516, whose disclosure is incorporated herein by reference. This patent describes a non-destructive method for testing the deposition and/or the removal of a material within a recess on the surface of a sample. An excitation beam is directed onto a region of the sample in a vicinity of the recess, and an intensity of X-ray fluorescence emitted from the region is measured. A quantity of the material that is deposited within the recess is determined responsively to the measured intensity.
U.S. Pat. No. 7,653,174, whose disclosure is incorporated herein by reference, describes methods for inspection of small features using X-ray fluorescence. These methods are based on measuring the intensity of X-ray emission from a sample at multiple different locations of an irradiating X-ray beam relative to a target feature on the sample. The corresponding intensity measurements are processed in order to give an adjusted value of the emission, which is more accurately indicative of characteristics (such as thickness) of the feature.
U.S. Patent Application Publication 2013/0089178, whose disclosure is incorporated herein by reference, describes a method for inspection of a feature formed on a semiconductor wafer, which includes a volume containing a first material and a cap made of a second material, different from the first material, that is formed over the volume. The feature is irradiated with a focused beam, and one or more detectors positioned at different angles relative to the feature are used to detect X-ray fluorescent photons that are emitted by the first material in response to the irradiating beam and pass through the cap before striking the detectors. Signals output by the one or more detectors at the different angles in response to the detected photons are processed in order to assess a quality of the cap.